IV pole hook design - Clarification 2

[BAC_eng1]

Hi Guys

I am working on a design for an IV pole hook which is required to carry 1.2kg bags. The challenge is to allow for a 20G frontal impact load factor. The design uses AL 6063-t6 rod - 6mm. The FEA indicates failure when the load moves slightly away from the fixed point. It passes when it is closer to the fixed point. I believe it is due to the moment increase. I advised this to the client, who is an electrical engineer and is also the manager for the system I am designing. He seems to be skeptical about it even after I showed him the FEA and my calculations. He reckons this design was used in the past with no issues and cannot believe 6mm AL rod would fail with only 240N. I am starting to doubt my own FEA, so I need clarification if I am doing the FEA correctly.

 

[3DDave]

It won't be T6 immediately after welding. Is there a heat-treat after welding?

I'm unclear as to how the bag on that hook sees 20gs. I can better picture the stand falling over and smashing into the floor which would included some of the energy from the bag on the other hook, that from the falling stand, and from the kinetic energy of the IV pump usually attached to such stands.

 

[BAC_eng1]

Hi 3DDave

Thanks for the quick reply. There is no heat treatment.

The standard that I am using stipulates 20G factor for static loads as the system is going to be used in ambulances and fixed winged aircrafts. Any load that goes on to the hooks is simply multiplied by that factor in the direction stipulated in the standard. At least that is my interpretation. It is from AS 4535 standard.

The preface to this standard state: “The basis for performance criteria set by this Standard is the restraint of multiple items of equipment and occupants during frontal impact under deceleration equivalent to 20g and a sideways impact under a deceleration equivalent to 10g.” Since there is no specific outline of the forces expected in the rearward direction, it is assumed using Appendix A, section A5 (c) (ii) that a 10g load would be applied in the rearward direction; “For sideways facing and rear facing compartment doors, the static force applied shall be equivalent to 10g deceleration.”

Is my interpretation incorrect?

 

[BAC_eng1]

I just realized that the bag restraint will fail before the AL hook fails. I have been so worried about just applying the load to the hook that I forgot the bags are made of plastic and that will fail before the hooks fails.

 

[3DDave]

Ah - OK. Similar requirements in airborne radar systems I worked on were to preclude items tearing loose and becoming projectiles. No one likes to barely survive a crash and get crushed by an equipment rack that broke loose.

The focus is on restraint, not to avoid deformation.

I argued, unsuccessfully, that the radar in the nose would not impact anyone, but it turned out de-rating the aluminum for fatigue dropped the allowables to amounts seen in 1-2 G vibration levels so it didn't matter.

 

[BAC_eng1]

Haha yep. I mean getting hit with a 1.2kg bag in the middle of a crash no matter how soft the projectile, is not ideal let alone getting hit with a radar.

It is easy to overlook because some of it seems counter-intuitive.

Also, do you reckon my reasoning is correct? I think the plastic bag will fail before the AL hooks. It just seems lazy to use that as a reasoning to pass the hooks as compliant. I couldn't find AL IV hooks anywhere online. So I am guessing the standard practice is to use SS rods instead.

 

[BAC_eng1]

Sorry just realised, the FEA indicates only yield, not ultimate failure. So I believe it should be ok.

 

[3DDave]

I am surprised at a choice to use aluminum. It is good when strength can be made with a larger volume of material, increasing the stiffness, but when the volume is constrained, such as by the standard hole size, steel is almost always the material of choice.

It's got to be in there somewhere about loads from sheddable payloads. I have no copy of the standard - the title suggest that it's to keep items from getting loose. I don't recall ever seeing an ambulance interior that constrained the bags other than by hooks. You'll need some AS/NZS 4535 expert to figure out how to deal with that in particular.

OTOH - the IV pole had better not come loose no matter what medical equipment gets hung from it, at least up to the 20gs.

 

[BAC_eng1]

I suggested SS just for the hooks. The hooks welded to a SS insert that just bolts in to AL pipe(the IV pole). They just like AL because it is lighter but for such a small component it doesn't make a difference.

I will look through the standards but you are right, I have not seen IV bags constrained other than by hooks.

Yes of course. I will be designing it and specifying a safe working height.

 

[CWB1]

I would take a deeper dive into your analysis as there's something fundamentally wrong there, units causing a misplaced decimal point perhaps? I also wouldn't assume the bag would fail first as they're pretty amazingly durable and even at 20Gs the force exerted is rather small. If in doubt and the hook is readily available, a fish scale can easily show whether/not you're going to fail this part on a bench.

 

[BAC_eng1]

I have an extremely fundamental question. The value you obtain from weight scales, is it in N? So if I weighed the IV bags on a weight scale and I get 1.2, does it reflect kgs or N?

 

[3DDave]

Many scales measure kilograms-weight, not kilograms-mass, and read the same value at 1 g. To find the force in Newtons to produce that accelerattion one must multiply by 1g or ~9.8 m/sec^2 times the number of g's expected. 1.2 *9.8 * 20 = ~240 N.

 

[BAC_eng1]

Ahh ok. The 240 N was correct then. Thank you.

 

[BAC_eng1]

Thank you guys! The part yields but does not fail. The design was correct, It was indicating failure because I had my material properties incorrectly saved.